Corrections and amplifications: This story has been updated to clarify the life expectancy of the new reflector plug, and that light water causes more neutrons to be absorbed than heavy water, rather than acting as the absorber.

Experiments are on hold at Oak Ridge National Laboratory's Spallation Neutron Source as crews replace the facility's inner reflector plug with a new one, filled with six metric tons of heavy water purchased from Iran.

Heavy water, or deuterium oxide, is a form of water that contains a larger-than-normal amount of the hydrogen isotope deuterium.

The water is called "heavy" because the deuterium increases the chemical's mass and affords it different properties than "light" water, which is the kind we drink.

The United States uses heavy water for nuclear research and weapons production. Dwindling supplies and an uncertain international market have made it an even hotter commodity.

The Department of Energy's last heavy water procurement came through a controversial Obama-era deal to get Iran back into compliance with a 2015 agreement it made with world powers to stop pursuing nuclear weapons.

More neutrons equals better science

Six tons of DOE's purchase will be used to upgrade Oak Ridge National Laboratory's Spallation Neutron Source.

The facility is the world's most powerful source of neutrons, allowing scientists to examine atomic reactions invisible to the human eye.

But it hasn't been producing as many neutrons as it could because until recently, an important component of the facility has been using light water instead of heavy water.

The way the neutron source works comes down to the facility's accelerator beams, which strip hydrogen atoms into protons, fire the protons across three football fields at 88 percent the speed of light, compress them and then slam them into a target made of mercury.

The bombardment causes the mercury in the target to spall, or break apart. (Thus, the name: Spallation Neutron Source.)

That's where the inner reflector plug comes in.

The plug's job is to keep the target cool when struck by a proton beam and reflect neutrons back to the target and its moderators.

A laboratory spokeswoman said the plug needed to be replaced anyway. She said it has been exposed to more than 40-gigawatt-hours worth of neutrons since the facility began operating in 2007.

Crews are filling a new plug with heavy water during an ongoing maintenance outage at the facility that began in December and will continue until June. Neutron scattering experiments will resume once the maintenance period is over.

The heavy water inside the new plug should last as long as the facility does, though the laboratory will keep a small reserve of heavy water to top it off every now and then. An ORNL spokeswoman said the plug itself will have to be replaced every five years.

The use of heavy water in the plug will maximize the target's neutron output.

The previous reflector plug was filled with light water which causes as much as 20 percent of the source's precious neutrons to be absorbed by cadmium in the plug, instead of reflecting the neutrons.

.That means fewer neutrons get put to work for high-impact experiments aimed at improving batteries, computer storage, and even cancer drugs.

The Iran Deal

The United States has not been able to produce heavy water since 1996, and the National Nuclear Security Administration has said resuming production will be costly and take at least 10 years of lead time.

So to maintain its programs, the Department of Energy has had to shop around.

At one point, Canada was a reliable supplier. But, DOE terminated a procurement process with the country when the Obama administration's interest in maintaining a 2015 nuclear agreement with Iran afforded DOE an alternative route.

In the agreement, world powers agreed to lift economic sanctions on Iran if it would halt its pursuit of nuclear weapons and allow continuous monitoring of its nuclear programs--including a heavy water production plant.

The country exceeded its 130-ton limit on heavy water, which it was required to sell, dilute or properly dispose of. In a controversial move, the United States bought the extra heavy water to get Iran back into compliance.

Critics of the move have pointed out the purchase could disrupt the international market and discourage the United States from pushing forward on a production plant of its own.

Mason said last year the United States was not planning to buy additional heavy water from Iran, echoing the Department of Energy's statement at the time of purchase.

DOE said that United States officials hoped the purchase would encourage other countries to buy Iran's excess heavy water for non-defense research in the future.

The United States can't use heavy water produced by foreign countries for its nuclear defense programs, so the Department of Energy is now in the heavy water sales business too.

A DOE spokesman could not be reached to specify how much of the extra 26 tons of heavy water have been repackaged and resold so far.